Heat energy into fuel?

[RenegadeRad]

Guys, a simple shower thought, I wanted to ask, is it possible to convert heat energy into some kind of fuel source for a spacecraft?

Imagine a scenario, where when the spacecraft re-enters orbit, the burn would produce so much heat to produce fuel source for it to slow down as a descend burn. Or, when aerobreaking on jupiter, a spacecraft would collect immense amount of heat and radiation enough for a return fuel?

Is there anything like this or am I thinking complete BS? Any science theory for this?

[lajoswinkler]

Heat is energy, but it can not exist on its own. Energy is carried by particles/fields (yes, photons aren't energy, they merely carry it). You could heat up a bunch of stuff and then expell it out like NERVA engine does...

[Sereneti]

energy to fuel :

E=mc²
yes it is, theoretically.

Practicaly: not..

But, why do you want to create fuel, if you have fuel already "lying around"?

you can use the atmosphere as "working mass"...

look at the ion engine...

 

[FungusForge]

If you have working mass of some kind, I don't see why not. Otherwise you'd be asking for some kind of mass fabricator to make the energy into something you can spit out of the back end of your rocket.

[kerbiloid]

1. It should require extremely powerful
heat exchanger (to cool the air from initial ~1500 K down to several hudred K on a distance of, say, 1-2 meters)
and radiators (to dissipate its waste heat from inside the 1000 K hot plasma cloud).

2. If the air drag is enough strong to heat your engines, probably you already need no engines at all. Anyway you can use them only to brake down — i.e. do the same as the drag force does.

3. Various spaceplane projects use the air gathering when flying-in or diving-into the upper atmosphere — as an oxidizer supply. But not a heat.

[Streetwind]

Assuming you had a working EMdrive (heh), you could use either thermocouples or a heat engine to generate electrical power from a heat source. That power could then be used to run the EMdrive.

All currently understood forms of propulsion (short of a photon drive) require fuel. To create fuel from nothing with just energy, you'd have to ask Mr. Einstein for advice, as has already been pointed out.

[p1t1o]

22 hours ago, RenegadeRad said:

Guys, a simple shower thought, I wanted to ask, is it possible to convert heat energy into some kind of fuel source for a spacecraft?

Imagine a scenario, where when the spacecraft re-enters orbit, the burn would produce so much heat to produce fuel source for it to slow down as a descend burn. Or, when aerobreaking on jupiter, a spacecraft would collect immense amount of heat and radiation enough for a return fuel?

Is there anything like this or am I thinking complete BS? Any science theory for this?

There are tons of ways that you can use energy to produce an energetic substance ("fuel", be it hydrogen/oxygen from water, hydrocarbons from feedstock, antimatter or whatever else you can come up with) from less energetic ones - the difficult part of your premise is having it compact, efficient and fast enough to fit on a ship that can use environmental heat to generate fuel in-situ.

But then fast-forward a couple of centuries, a waste-heat scavenging fuel cell might be what powers your phone

NB: In *theory*, all chemical reactions are reversible. So, in *theory* (and you'll soon notice why I stress that part so much) you could reverse an entire rocket launch. If you organise a column of atmosphere with the end products of a rocket exhaust, with every particle moving backwards relative to the path of a normal burned-and-exhausted particles. So you have a column of hot combustion products that you have made to quickly contract, if you get the speed and vector of each particle correct, you should be able to drop an empty rocket out of orbit, and if it is on precisely the correct path, the rocket nozzle will collect all of the particles that are zooming towards it (which are now heated to incandescence from compression effects) filling the combustion chamber, where the products will get so hot that they split back into fuel+oxidiser and the particles of that should (if you get everything *precisely* right), will zoom into the fuel/oxidiser injectors and fill up your tanks with cold liquid fuel (the process of splitting the combustion products back to fuel/oxidiser will be strongly endothermic and will chill your incandescent plasma exhaust back down)

Note: this is of course impossible, unless you have the technological prowess of a god.

[peadar1987]

Unfortunately, that's not possible, even in theory. We've all probably heard of the laws of thermodynamics. We were given this alternative, tongue in cheek version in a lecture:

-You can never win, you can only break even

-You can only break even at absolute zero

-You can never reach absolute zero

Basically, the second law of thermodynamics says that you can never convert 100% of thermal energy in a heat source into useful work unless you have an infinitely large temperature gradient. You always lose some of it as low-grade heat. The bigger the ratio of temperatures in the system, the better you can do (and rocket engines do particularly well, as the chamber temperature is extremely high in relation to ambient conditions), however, you still lose about 10-20% of the energy as unusable waste heat, which means the process is never completely reversible.

Edited May 31, 2016 by peadar1987

[RCgothic]

You could never recover all the energy from an exhaust to create new fuel. It's not a reversible process, as has been discussed.

You could scavenge temperature from other sources (solar, aerobraking) to accelerate a propellant or fabricate a fuel from component raw materials. 

But heat gives very poor return as an energy source compared to chemical sources, and you'd still need to carry a propellant or fuel materials with you. It would be easier to just bring fuel.

 

This isn't really practical for a craft that doesn't spend a long time scavenging energy and propellant mass.

[todofwar]

20 minutes ago, peadar1987 said:

Unfortunately, that's not possible, even in theory. We've all probably heard of the laws of thermodynamics. We were given this alternative, tongue in cheek version in a lecture:

-You can never win, you can only break even

-You can only break even at absolute zero

-You can never reach absolute zero

Basically, the second law of thermodynamics says that you can never convert 100% of thermal energy in a heat source into useful work unless you have an infinitely large temperature gradient. You always lose some of it as low-grade heat. The bigger the ratio of temperatures in the system, the better you can do (and rocket engines do particularly well, as the chamber temperature is extremely high in relation to ambient conditions), however, you still lose about 10-20% of the energy as unusable waste heat, which means the process is never completely reversible.

Or the even more succinct: you can't win, you can't break even, you can't get out of the game. 

I had an idea for placing your periapse just low enough to compress hydrogen in front of you without heating it too much (you will heat it because you're compressing it, obviously, but if you're shallow enough maybe not to plasma levels) so you can collect it into your tank. With some thermoelectrics you could even capture some of the heat of the compressed hydrogen and you have yourself fuel and energy for an ion drive. So, not just the heat but capturing the gas as well might work.

Of course, that's not going to be the best way to get energy since thermoelectrics are only like 20% efficient on a good day. 

[p1t1o]

25 minutes ago, peadar1987 said:

Unfortunately, that's not possible, even in theory. We've all probably heard of the laws of thermodynamics. We were given this alternative, tongue in cheek version in a lecture:

-You can never win, you can only break even

-You can only break even at absolute zero

-You can never reach absolute zero

Basically, the second law of thermodynamics says that you can never convert 100% of thermal energy in a heat source into useful work unless you have an infinitely large temperature gradient. You always lose some of it as low-grade heat. The bigger the ratio of temperatures in the system, the better you can do (and rocket engines do particularly well, as the chamber temperature is extremely high in relation to ambient conditions), however, you still lose about 10-20% of the energy as unusable waste heat, which means the process is never completely reversible.

Well you would also have to provide all of the radiated energy as well as the particle movements. I was under the impression that perfectly time-reversed (including reversing all waste heat and other thermodynamic and entropic losses) reactions should be "possible"? But yeah I wouldn't be surprised if thermodynamics, or the uncertainty principle, forbids this, it was just a neat thought.

[PB666]

The velocity of a gas is dependent on its molecular weight and the temperature. In most cases it takes very high temperatures to reach a reasonable ISP. If we argue that IR has a gas temperature of about 2500K thats a rather puny ISP.  If you really want to get the ISP up you have to convert the heat to electricity, in which case you lose half of the energy, at least, in order to create a gradient, presumbably with space. With the remaining half you create a low voltage wavefunction, then transform it into a high voltage wave function, whose waste heat you could scavenge. From there you have to use a rectifyer to create a high voltage, you also need a UV light/X ray ion generator. At which point you ionize particles then place the cations in a electric field and accelerate them to 10000s of m/s. This is how a fission reactor based ion drive system works. You can use sunlight as a source of heat, but again it has a maximum ISP because of the average 'temperature' of light coming from the sun. You can grab some of this with solar panels and use this to run transformers that run the ion drive systems. A third, rather messy option is to take antimatter and matter, let them react, and then convert the high energy photons into lower energy photons using a number of high energy gamma absorbers, this will tend to create alot of nuclear byproducts, which in turn will heat up and produce things like neutrons, etc, and eventually you have heat and a very trashed up reactor lining. 

More sophisticated versions of heat generation are fusion reactors (although technically once it gets going it may actually produce more light and x-rays than heat). Another method is light channelling, where you have some system of gathering light in the inner solar system, and creating channels that provide heat and light to the outer solar system whereby spacecraft can feed, stock pile energy and go about the business they need to do. This in mid/far future.

 

[pincushionman]

"Heat" (or more what is happening in a reentry situation, "being really hot") is not, by itself, very useful. Without a temperature differential to go along with it, it's pretty much worthless. And because of conduction in your device any temperature differential you might get by absorbing heat from that kind if environment is going to go away sooner rather than later, so "storing" it for any length of time seems like a bit of a fool's errand.